Sintered tungsten alloy

ABSTRACT

There is provided a flat plate-like sintered tungsten alloy that can be molded into a complex shape by press working or forge processing. The flat plate-like sintered tungsten alloy contains 85% by mass or more and 98% by mass or less of W, 1.4% by mass or more and 11% by mass or less of Ni, and 0.6% by mass or more and 6% by mass or less of at least one substance selected from the group consisting of Fe, Cu and Co, wherein an elongation percentage of the flat plate-like sintered tungsten alloy in a planar direction is 20% or more.

TECHNICAL FIELD

The present invention generally relates to a sintered tungsten alloy,and particularly to a sintered tungsten alloy used as a radiation shieldmaterial in radiation medical devices, nuclear reactor-related devicesand the like.

BACKGROUND ART

It is conventionally known to use, as a radiation shield material, atungsten-based alloy material containing tungsten as a main component.

Japanese Patent Laying-Open No. 9-71828 (hereinafter referred to as “PTD1”), for example, discloses a tungsten-based alloy material forradiation shielding, which has a layered structure obtained byperforming plastic working on a sintered body containing 85% by weightor more of tungsten as a main component, with the rest consisting ofnickel and iron or copper, and flattening tungsten particles and abinder layer containing nickel, and layering these flattened layers.This tungsten-based alloy material is obtained by sintering a moldedbody at 1470° C. to form a sintered body, and rolling the obtainedsintered body at a heating temperature of 1300° C. such that aprocessing rate in total becomes about 60%, and flattening the tungstenparticles and the binder layer.

Japanese Patent Laying-Open No. 9-235641 discloses a tungsten heavyalloy plate containing tungsten at a weight ratio of 80 to 97%, nickelat a weight ratio of 2 to 15%, and one or two or more of iron, copperand cobalt at a weight ratio of 1 to 10% in total, having a thickness of0.3 mm or smaller, and having such a dimension that both sides are morethan 200 times as large as the thickness. This heavy alloy plate isobtained by mixing material powders, molding the mixture into a thinplate having a thickness of 0.35 mm or smaller by a powder rollingpress, and then, sintering the molded body in the non-oxidizingatmosphere, hot-rolling and/or cold-rolling the sintered body as needed,and then, performing finish rolling for flattening and smoothing.

CITATION LIST Patent Document

-   PTD 1: Japanese Patent Laying-Open No. 9-71828-   PTD 2: Japanese Patent Laying-Open No. 9-235641

SUMMARY OF INVENTION Technical Problem

As disclosed in PTD 1, the sintered tungsten alloy containing 85% bymass or more of tungsten has the radiation shielding effect, and thus,is used as a radiation shield material in radiation medical devices,nuclear reactor-related devices and the like. When the sintered tungstenalloy is used in such an application, it is necessary to fabricate aflat plate-like sintered tungsten alloy having a large area of a certaindegree or more.

However, the conventional sintered tungsten alloy is not sufficient inelongation as a material property. Therefore, there is a problem that aflat plate-like radiation shield member having a complex shape cannot bemolded by press working or forge processing.

Thus, an object of the present invention is to provide a flat plate-likesintered tungsten alloy that can be molded into a complex shape by pressworking or forge processing.

Solution to Problem

A sintered tungsten alloy according to the present invention is a flatplate-like sintered tungsten alloy, containing 85% by mass or more and98% by mass or less of tungsten, 1.4% by mass or more and 11% by mass orless of nickel, and 0.6% by mass or more and 6% by mass or less of atleast one substance selected from the group consisting of iron, copperand cobalt. An elongation percentage of the flat plate-like sinteredtungsten alloy in a planar direction is 20% or more.

Preferably, in the sintered tungsten alloy according to the presentinvention, a thickness of the filat plate-like sintered tungsten alloyis 1.5 mm or smaller.

Preferably, in the sintered tungsten alloy according to the presentinvention, an X-ray diffraction intensity ratio of a (111) plane of aNi—(Fe, Cu, Co) phase in a flat plate surface of the flat plate-likesintered tungsten alloy (assuming that X-ray diffraction intensities ofthe (111) plane, a (100) plane, a (110) plane, and a (311) plane areI(111), I(100), I(110), and I(311), respectively, the X-ray diffractionintensity ratio of the (111) plane is a value of[I(111)/{I(111)+I(100)+I(110)+I(311)}]) is 0.68 or more and 0.9 or less.

Advantageous Effects of Invention

According to the present invention, there can be provided a flatplate-like sintered tungsten alloy that can be molded into a complexshape by press working or forge processing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an optical microscope photograph showing a cross section of aflat plate-like sintered tungsten alloy after a sintering step isperformed during fabrication in Example 1 of the present invention.

FIG. 2 is an optical microscope photograph showing a cross section ofthe flat plate-like sintered tungsten alloy after a distortionintroducing step and a heat treatment step are performed duringfabrication in Example 1 of the present invention.

FIG. 3 is an optical microscope photograph showing a cross section ofthe flat plate-like sintered tungsten alloy fabricated in Example 1 ofthe present invention.

FIG. 4 is an optical microscope photograph showing a cross section of aflat plate-like sintered tungsten alloy fabricated in ComparativeExample 2 of the present invention.

FIG. 5 is a schematic perspective view showing two cross sectionsobserved in the flat plate-like sintered tungsten alloy fabricated as anintermediate product in Examples and Comparative Examples of the presentinvention.

FIG. 6 is a scanning electron microscope (SEM) photograph showing across section portion observed in the flat plate-like sintered tungstenalloy fabricated as an intermediate product in Example 1 of the presentinvention.

FIG. 7 is a diagram schematically showing the measured thicknesses andlengths of tungsten crystal grains at the cross section portion observedin the flat plate-like sintered tungsten alloy fabricated as anintermediate product in Examples and Comparative Examples of the presentinvention.

FIG. 8(A) is a plan view and FIG. 8(B) is a side view showing thedimension of a tensile test piece fabricated from the flat plate-likesintered tungsten alloy as a final product fabricated in Examples andComparative Examples of the present invention.

DESCRIPTION OF EMBODIMENTS

A flat plate-like sintered tungsten alloy of the present invention as anintermediate product contains 85% by mass or more and 98% by mass orless of tungsten (W), 1.4% by mass or more and 11% by mass or less ofnickel (Ni), and 0.6% by mass or more and 6% by mass or less of at leastone substance selected from the group consisting of iron (Fe), copper(Cu) and cobalt (Co). This sintered tungsten alloy has such a structurethat a plurality of flattened tungsten crystal grains extending alongthe extending direction of a plane of the flat plate-like sinteredtungsten alloy are stacked. A first cross section along the thicknessdirection orthogonal to the extending direction of the plane of the flatplate-like sintered tungsten alloy and a second cross section which is across section along the thickness direction orthogonal to the extendingdirection of the plane of the flat plate-like sintered tungsten alloyand which is orthogonal to the first cross section are observed toobtain a first cross section portion of a certain width and a certainthickness selected from the first cross section and a second crosssection portion of a certain width and a certain thickness selected fromthe second cross section. A ratio of an average length to an averagethickness of a plurality of tungsten crystal grains which are observedat the first cross section portion and the second cross section portionand which intersect with a center line passing through a center of thecertain width and extending in the direction of the certain thickness is9 or more and 125 or less.

Since the ratio of the average length to the average thickness of thetungsten crystal grains observed as described above is 9 or more and 125or less in the flat plate-like sintered tungsten alloy as anintermediate product configured as described above, higher strength canbe obtained at high temperature than that of the conventional sinteredtungsten alloy. If the ratio of the average length to the averagethickness of the tungsten crystal grains observed as described above isless than 9, there is a possibility that sufficiently high strengthcannot be obtained at high temperature. If the ratio of the averagelength to the average thickness of the tungsten crystal grains observedas described above exceeds 125, fracture may occur.

In order to achieve the aforementioned ratio, the average thickness ofthe tungsten crystal grains observed as described above is preferably 2μm or larger and 10 μm or smaller, and the average length of thetungsten crystal grains observed as described above is preferably 30 μmor longer and 250 μm or shorter. It is difficult to set the averagethickness of the tungsten crystal grains to be smaller than 2 μm or toset the average length to exceed 250 μm.

If the average thickness of the tungsten crystal grains observed asdescribed above is 2 μm or larger and 3 μm or smaller, the averagelength of the tungsten crystal grains observed as described above ispreferably within a range of 30 μm to 250 μm, and the ratio of theaverage length to the average thickness is preferably 10 or more and 125or less.

If the average thickness of the tungsten crystal grains observed asdescribed above exceeds 3 m and is 6 μm or smaller, the average lengthof the tungsten crystal grains observed as described above is preferablywithin a range of 54 μm to 250 μm, and the ratio of the average lengthto the average thickness is preferably 9 or more and 84 or less, inorder to obtain sufficiently high strength at high temperature.

Furthermore, if the average thickness of the tungsten crystal grainsobserved as described above exceeds 6 μm and is 10 μm or smaller, theaverage length of the tungsten crystal grains observed as describedabove is preferably within a range of 90 μm to 250 μm, and the ratio ofthe average length to the average thickness is preferably 9 or more and42 or less, in order to obtain sufficiently high strength at hightemperature.

The tungsten crystal grain in the planar direction of the flatplate-like sintered tungsten alloy has various shapes such as asubstantially circular shape, a substantially oval shape, asubstantially square shape, a substantially rectangular shape, and anindefinite shape.

The flat plate-like sintered tungsten alloy of the present invention asan intermediate product is manufactured through (1) materials preparingstep, (2) mixing step, (3) molding step, (4) sintering step, (5)distortion introducing step, (6) heat treatment step, and (7)hot-rolling step.

In short, a method for manufacturing the flat plate-like sinteredtungsten alloy of the present invention as an intermediate product is amethod for manufacturing a sintered tungsten alloy containing 85% bymass or more and 98% by mass or less of tungsten, 1.4% by mass or moreand 11% by mass or less of nickel, and 0.6% by mass or more and 6% bymass or less of at least one substance selected from the groupconsisting of iron, copper and cobalt, characterized in that adistortion is introduced into a sintered body and heat treatment isperformed on the sintered body having the distortion introduced therein,and then, the sintered body is hot-rolled at a rolling processing rateof 60% or more.

The flat plate-like sintered tungsten alloy thus manufactured can havesuch a structure that the ratio of the average length to the averagethickness of the tungsten crystal grains observed as described above is9 or more and 125 or less, and higher strength can be obtained at hightemperature than that of the conventional sintered tungsten alloy.

In particular, a certain level of distortion is provided to a sinteredtungsten alloy (sintered body) that has tungsten crystal grains with agrain size of 30 to 50 μm fabricated in accordance with a conventionallyknown manufacturing method (a mixing step, a molding step and asintering step of material powders), and then, a certain level of heattreatment is performed. As a result, the grain size of the tungstencrystal grain in the sintered tungsten alloy can be decreased to acertain value or smaller (5 to 20 μm). Thereafter, the sintered body ishot-rolled at a rolling processing rate of 60% or more. As a result, thethickness of the tungsten crystal grain can be decreased to a certainvalue or smaller and the length of the tungsten crystal grain can beincreased to a certain value or larger, and thus, there can be obtaineda sintered tungsten alloy having such a structure that a plurality offlattened tungsten crystal grains extending in the planar direction ofthe flat plate-like sintered tungsten alloy are stacked. As a result,higher strength can be obtained at high temperature than that of theconventional sintered tungsten alloy.

The flat plate-like sintered tungsten alloy according to the presentinvention contains 85% by mass or more and 98% by mass or less oftungsten, 1.4% by mass or more and 11% by mass or less of nickel, and0.6% by mass or more and 6% by mass or less of at least one substanceselected from the group consisting of iron, copper and cobalt. Anelongation percentage of the flat plate-like sintered tungsten alloy inthe planar direction is 20% or more.

Since the elongation percentage of the flat plate-like sintered tungstenalloy in the planar direction is 20% or more in the flat plate-likesintered tungsten alloy configured as described above, the flatplate-like sintered tungsten alloy can be molded into a complex shape bypress working or forge processing.

If the elongation percentage of the flat plate-like sintered tungstenalloy in the planar direction is less than 20%, there is no differencein elongation percentage from the conventional sintered tungsten alloyand there is a possibility that the flat plate-like sintered tungstenalloy cannot be molded into a complex shape by press working or forgeprocessing. An upper limit value of the elongation percentage of theflat plate-like sintered tungsten alloy in the planar direction is 45%.It is difficult to obtain a flat plate-like sintered tungsten alloy inwhich the elongation percentage of the flat plate-like sintered tungstenalloy in the planar direction exceeds 45%.

The flat plate-like sintered tungsten alloy according to the presentinvention is manufactured through (1) materials preparing step, (2)mixing step, (3) molding step, (4) sintering step, (5) distortionintroducing step, (6) heat treatment step, (7) hot-rolling step, and (8)heat treatment step.

In short, the method for manufacturing the flat plate-like sinteredtungsten alloy according to the present invention is a method formanufacturing a sintered tungsten alloy containing 85% by mass or moreand 98% by mass or less of tungsten, 1.4% by mass or more and 11% bymass or less of nickel, and 0.6% by mass or more and 6% by mass or lessof at least one substance selected from the group consisting of iron,copper and cobalt, characterized in that a distortion is introduced intoa sintered body and heat treatment is performed on the sintered bodyhaving the distortion introduced therein, and then, the sintered body ishot-rolled at a rolling processing rate of 60% or more and further heattreatment is performed on the hot-rolled sintered body.

In the flat plate-like sintered tungsten alloy thus manufactured, theelongation percentage of the flat plate-like sintered tungsten alloy inthe planar direction can be 20% or more, and the flat plate-likesintered tungsten alloy can be molded into a complex shape by pressworking or forge processing.

In particular, the flat plate-like sintered tungsten alloy according tothe present invention is obtained by performing further heat treatmenton the flat plate-like sintered tungsten alloy of the present inventionas an intermediate product. By this heat treatment, the flat plate-likesintered tungsten alloy does not have the structure of the sinteredtungsten alloy of the present invention as an intermediate product, buthas a structure very close to a structure of the conventional sinteredtungsten alloy immediately after sintering. However, the sinteredtungsten alloy according to the present invention is returned throughthe structure of the sintered tungsten alloy of the present invention asan intermediate product to the structure very close to the structure ofthe conventional sintered tungsten alloy. As a result, almost alldefects present at an interface between the tungsten crystal grains andthe binder (nickel, iron and the like) and in the binder, which are acause of reduction in elongation in the conventional sintered tungstenalloy, disappear, and thus, it becomes easy for the tungsten crystalgrains to slide in the binder component. Consequently, elongation of thesintered tungsten alloy according to the present invention is enhanced.

Preferably, the thickness of the flat plate-like sintered tungsten alloyaccording to the present invention is 1.5 mm or smaller.

Preferably, in the sintered tungsten alloy according to the presentinvention, an X-ray diffraction intensity ratio of a (111) plane of aNi—(Fe, Cu, Co) phase in a flat plate surface of the flat plate-likesintered tungsten alloy (assuming that X-ray diffraction intensities ofthe (111) plane, a (100) plane, a (110) plane, and a (311) plane areI(111), I(100), I(110), and I(311), respectively, the X-ray diffractionintensity ratio of the (111) plane is a value of[I(111)/{I(111)+I(100)+I(110)+I(311)}]) is 0.68 or more and 0.9 or less.

A reason why the X-ray diffraction intensity ratio of the (111) plane ofthe Ni—(Fe, Cu, Co) phase in the flat plate surface of the flatplate-like sintered tungsten alloy of the present invention becomes highas described above, and the function and effect produced by this can bedescribed as follows. When the flattened structure of the tungstencrystal grains having an extremely high aspect ratio is formed in (7)hot-rolling step described above, the Ni—(Fe, Cu, Co) phase serving as abinder phase is also stretched in the planar direction. As a result, inthe Ni—(Fe, Cu, Co) phase serving as a binder phase, the (111) planeserving as a sliding plane of an FCC (face-centered cubic) structure isoriented in parallel to the planar direction. This has an influence evenafter (8) heat treatment step described above, and the orientation ofthe (111) plane remains in the flat plate-like sintered tungsten alloyas a final product. Since the sliding plane of the binder plane isoriented in parallel to the planar direction as described above, it canbecome easy for the tungsten crystal grains to slide in the bindercomponent. By heat treatment, almost all defects present in the sinteredtungsten alloy can be eliminated and elongation of the sintered tungstenalloy in the planar direction can be further enhanced.

In the conventional sintered tungsten alloy, warpage or distortionoccurs, and thus, it is difficult to form a thin flat plate immediatelyafter sintering. In addition, when rolling processing is performed onthe conventional sintered tungsten alloy at a rolling processing rate of60% or more, fracture and the like occur, and thus, it is necessary tofinally perform grinding and polishing in order to obtain a thin flatplate. As a result, the manufacturing cost becomes high.

In contrast, in the method for manufacturing the sintered tungsten alloyaccording to the present invention, a distortion is introduced into thesintered body and heat treatment is performed on the sintered bodyhaving the distortion introduced therein, and then, hot-rollingprocessing is performed. Therefore, the sintered body can be hot-rolledat a rolling processing rate of 60% or more. As a result, the flatplate-like sintered tungsten alloy having a thickness of 1.5 mm orsmaller can be formed. Therefore, the manufacturing method of thepresent invention is particularly advantageous to obtain a thin flatplate and the manufacturing cost can be reduced.

In particular, in the method for manufacturing the sintered tungstenalloy according to the present invention, further heat treatment isperformed after hot-rolling processing, and thus, elongation can beenhanced. Therefore, the obtained flat plate-like sintered tungstenalloy can be further thinned by rolling processing and the like. A lowerlimit value of the thickness of the flat plate-like sintered tungstenalloy according to the present invention is 0.05 mm. It is difficult toset the thickness of the flat plate-like sintered tungsten alloy to besmaller than 0.05 mm.

The sintered tungsten alloy according to the present invention maycontain elements other than nickel (Ni), iron (Fe), copper (Cu), andcobalt (Co), and may contain, for example, 0% by mass or more and 2% bymass or less of an element such as manganese (Mn), molybdenum (Mo),silicon (Si), rhenium (Re), chromium (Cr), titanium (Ti), vanadium (V),niobium (Nb), and tantalum (Ta), to such an extent that the function andeffect of the present invention are not impaired.

The method for manufacturing the flat plate-like sintered tungsten alloyof the present invention will be described hereinafter.

(1) Materials Preparing Step

A tungsten powder, a nickel powder, and a metal powder of at least onesubstance selected from the group consisting of iron, copper and cobaltare prepared. The materials are prepared to contain the tungsten powderat a blending ratio of 85% by mass or more and 98% by mass or less,nickel at a blending ratio of 1.4% by mass or more and 11% by mass orless, and the metal powder of at least one substance selected from thegroup consisting of iron, copper and cobalt at a blending ratio of 0.6%by mass or more and 6% by mass or less.

If the blending ratio of the tungsten powder is less than 85% by mass,the strength of the obtained sintered tungsten alloy may beinsufficient. If the blending ratio of the tungsten powder exceeds 98%by mass, the binder component may be insufficient and fracture may occurin the obtained sintered tungsten alloy during the rolling step.

If the blending ratio of the nickel powder is less than 1.4% by mass,fracture may occur in the obtained sintered tungsten alloy during therolling step. If the blending ratio of the nickel powder exceeds 11% bymass, the strength of the obtained sintered tungsten alloy may beinsufficient.

Iron, copper and cobalt serve as a sintering aid. If the blending ratioof the metal powder of at least one substance selected from the groupconsisting of iron, copper and cobalt is less than 0.6% by mass, thedense sintered body cannot be obtained, and thus, fracture may occur inthe obtained sintered tungsten alloy during the rolling step. If theblending ratio of the aforementioned metal powder exceeds 6% by mass,the binder component is excessively cured, and thus, the toughness ofthe obtained sintered tungsten alloy itself may be reduced.

The average grain size of each of the tungsten powder, the nickel powderand the aforementioned metal powder is preferably 1 μm or larger and 10μm or smaller. If the average grain size of each of these powders issmaller than 1 μm, the manufacturing cost may increase. If the averagegrain size of each of these powders exceeds 10 μm, a gap is likely to beformed in the obtained sintered tungsten alloy, and thus, fracture mayoccur in the sintered tungsten alloy during the rolling step.

(2) Mixing Step

The tungsten powder, the nickel powder, and the metal powder of at leastone substance selected from the group consisting of iron, copper andcobalt, which were prepared as described above, are mixed to obtain amixture. Mixing can be performed by using a Lodige mixer, an attritor, aball mill or the like. At the time of mixing, a solvent and a binder maybe added to the material powders. Camphor, Merball, stearic acid,paraffin or the like can be used as the binder. Ethanol, methanol,acetone or the like can be used as the solvent.

(3) Molding Step

Pressure is applied to the mixture obtained as described above and themixture is molded to obtain a molded body. The pressure is applied tothe mixture by using cold isotropic pressure press (CIP), dry CIP,mechanical press or the like. The pressure applied at the time ofmolding is preferably 49 MPa or higher and lower than 294 MPa. If thepressure is lower than 49 MPa, there is a possibility that the moldedbody cannot be obtained. Even if the molded body can be obtained, themolded body may be broken during handling and the subsequent steps. Evenif the pressure is 294 MPa or higher, no problem arises, while thefunction for obtaining the molded body is not enhanced.

(4) Sintering Step

The molded body obtained as described above is sintered to obtain asintered body. The atmosphere of a hydrogen gas, vacuum or an inert gascan be used as the atmosphere for sintering the molded body. A batchfurnace, a continuous pressure furnace or the like can be used as asintering furnace that houses the molded body.

The sintering temperature is preferably 1200° C. or higher and 1550° C.or lower. If the sintering temperature is lower than 1200° C., the densesintered body cannot be obtained, and thus, fracture may occur in theobtained sintered tungsten alloy during the rolling step. If thesintering temperature exceeds 1550° C., the sintered body may melt.

The sintering time is preferably 10 minutes or longer and 300 minutes orshorter when the sintering temperature is maximum. If the sintering timeis shorter than 10 minutes, the dense sintered body cannot be obtained,and thus, fracture may occur in the obtained sintered tungsten alloyduring the rolling step. If the sintering time exceeds 300 minutes, thetungsten crystal grains become too coarse, and thus, there is apossibility that the tungsten crystal grains cannot become sufficientlyfine grains during the subsequent steps.

The molding step and the sintering step may be simultaneously performedby using hot isotropic pressure press (HIP). In this case, an inert gassuch as a nitrogen gas and an argon gas can be used as the atmosphere.

The pressure is preferably 500 MPa or higher and 1500 MPa or lower. Ifthe pressure is lower than 500 MPa, the dense sintered body cannot beobtained, and thus, fracture may occur in the obtained sintered tungstenalloy during the rolling step. Even if the pressure exceeds 1500 MPa, noproblem arises, while the function for obtaining the sintered body isnot enhanced.

The temperature is preferably 1200° C. or higher and 1550° C. or lower.If the temperature is lower than 1200° C., the dense sintered bodycannot be obtained, and thus, fracture may occur in the obtainedsintered tungsten alloy during the rolling step. If the temperatureexceeds 1550° C., the sintered body may melt.

(5) Distortion Introducing Step

A distortion is introduced into the sintered body (sintered tungstenalloy) obtained as described above. The distortion is preferablyintroduced into the sintered body by, for example, deforming the flatplate-like sintered body in the thickness direction at a deformationrate of 20% or more and 50% or less. If the deformation rate is lessthan 20%, an amount of distortion introduced into the sintered body isinsufficient, and thus, there is a possibility that fine crystal grainscannot be formed even if heat treatment is performed in the subsequentstep. If the deformation rate exceeds 50%, fracture may occur in thesintered body.

The temperature of the sintered body at the time of introduction of thedistortion is preferably 0° C. or higher and 600° C. or lower. If thetemperature is lower than 0° C., the sintered body becomes hard, andthus, fracture may occur. If the temperature exceeds 600° C., theintroduced distortion is released, and thus, there is a possibility thatfine crystal grains cannot be formed even if heat treatment is performedin the subsequent step.

A method for introducing the distortion into the sintered body caninclude deforming the sintered body by forge processing, mechanicalpress working, cold-rolling processing or the like.

(6) Heat Treatment Step

Heat treatment is performed on the sintered body (sintered tungstenalloy) having the distortion introduced therein. By this heat treatment,the distortion introduced into the sintered body is recovered to anappropriate level and the tungsten crystal grains become fine grains.

The atmosphere of vacuum, a hydrogen gas, a nitrogen gas, an argon gas,or a carbon monoxide gas can be used as the atmosphere for performingheat treatment on the sintered body.

The heat treatment temperature is preferably 900° C. or higher and 1400°C. or lower. If the heat treatment temperature is lower than 900° C.,recovery of the distortion is insufficient and the tungsten crystalgrains do not become fine grains, and thus, fracture may occur in thesintered body during the subsequent hot-rolling step. If the heattreatment temperature exceeds 1400° C., the distortion is completelyrecovered and the tungsten crystal grains become coarse, and thus,fracture may occur in the sintered body during the subsequenthot-rolling step.

The heat treatment time is preferably 20 minutes or longer and 5 hoursor shorter. If the heat treatment time is shorter than 20 minutes,recovery of the distortion is insufficient and the tungsten crystalgrains do not become fine grains, and thus, fracture may occur in thesintered body during the subsequent hot-rolling step. If the heattreatment time exceeds 5 hours, the distortion is completely recoveredand the tungsten crystal grains become coarse, and thus, fracture mayoccur in the sintered body during the subsequent hot-rolling step.

The average grain size of the tungsten crystal grains in the sinteredtungsten alloy after heat treatment is preferably 5 μm or larger and 20μm or smaller. It is difficult to set the average grain size of thetungsten crystal grains to be smaller than 5 μm. If the average grainsize of the tungsten crystal grains exceeds 20 μm, fracture may occur inthe sintered body during the subsequent hot-rolling step.

(7) Hot-Rolling Step

Rolling processing is performed on the sintered body (sintered tungstenalloy) subjected to heat treatment as described above at a rollingprocessing rate of 60% or more, with the sintered body heated. Theatmosphere of a hydrogen gas, a nitrogen gas, an argon gas or the likecan be used as the atmosphere for heating the sintered body.

The rolling processing temperature is preferably 800° C. or higher and1400° C. or lower. If the rolling processing temperature is lower than800° C., a large load is applied to a rolling machine, and thus, rollingprocessing cannot be performed or fracture may occur in the sinteredbody. If the rolling processing temperature exceeds 1400° C., thetungsten crystal grains become coarse, and thus, fracture may occur inthe sintered body when rolling processing is performed.

The rolling processing rate of one rolling is preferably 5% or more and30% or less. If the rolling processing rate of one rolling is less than5%, the number of rolling for achieving the rolling processing rate of60% or more in total increases and the manufacturing cost increases. Ifthe rolling processing rate of one rolling exceeds 30%, a large load isapplied to the rolling machine, and thus, rolling processing cannot beperformed or fracture may occur in the sintered body.

The rolling processing rate in total is preferably 60% or more and 95%or less. If the rolling processing rate in total is less than 60%, thetungsten crystal grains do not become flattened particles, and thus,there is a possibility that sufficiently high strength of the sinteredtungsten alloy at high temperature is not obtained. If the rollingprocessing rate in total exceeds 95%, fracture may occur in the sinteredbody due to rolling processing.

Even when (5) distortion introducing step and (6) heat treatment stepare not performed on the conventional sintered tungsten alloy and (7)hot-rolling step is performed on the sintered tungsten alloy immediatelyafter sintering, rolling is performed at a rolling processing rate ofapproximately 60% at most. Even when the hot-rolling step is performedon the flat plate-like sintered tungsten alloy having, for example, aplane of approximately 100 mm×100 mm or larger, the sintered tungstenalloy having a thickness of approximately 2 mm is only obtained.

In contrast, in the present invention, (5) distortion introducing stepand (6) heat treatment step are performed and then (7) hot-rolling stepis performed on the sintered body (sintered tungsten alloy) obtained in(4) sintering step. As a result, rolling can be performed at a rollingprocessing rate of 60% or more and 95% or less and the flat plate-likesintered tungsten alloy as an intermediate product having a thickness of1.5 mm or smaller can be manufactured. A lower limit value of thethickness of the sintered tungsten alloy of the present invention as anintermediate product is 0.5 mm.

It is difficult to obtain the sintered tungsten alloy having a thicknessof smaller than 0.5 mm, even when (5) distortion introducing step and(6) heat treatment step are performed and then (7) hot-rolling step isperformed.

(8) Heat Treatment Step

Heat treatment is performed on the sintered body (sintered tungstenalloy) subjected to hot-rolling processing as described above. By thisheat treatment, almost all defects present at the interface between thetungsten crystal grains and the binder and in the binder can beeliminated, and thus, it becomes easy for the tungsten crystal grains toslide in the binder component. Consequently, elongation of the sinteredtungsten alloy is enhanced.

The atmosphere of vacuum, a hydrogen gas or an inert gas can be used asthe atmosphere for performing heat treatment on the sintered bodysubjected to hot-rolling processing. A batch furnace or a continuouspressure furnace can be used as a furnace for performing heat treatment.

The heat treatment temperature is preferably 1300° C. or higher and1550° C. or lower. If the heat treatment temperature is lower than 1300°C., the tungsten crystal grains become coarse insufficiently, and thus,there is a possibility that the processability in cold working is notenhanced. If the heat treatment temperature exceeds 1550° C., thesintered body may melt.

The heat treatment time is preferably 10 minutes or longer and 5 hoursor shorter. If the heat treatment time is shorter than 10 minutes, thetungsten crystal grains become coarse insufficiently, and thus, there isa possibility that the processability in cold working is not enhanced.If the heat treatment time exceeds 5 hours, the tungsten crystal grainsbecome coarse and the processability decreases, and thus, fracture mayoccur in the sintered body when the sintered body is processed into acomplex shape.

The average grain size of the tungsten crystal grains in the sinteredtungsten alloy after heat treatment is preferably 20 μm or larger and 60μm or smaller. If the average grain size of the tungsten crystal grainsis smaller than 20 μm, the effect of heat treatment is insufficient andhigh elongation percentage is not obtained, and thus, there is apossibility that the processability in cold working is not enhanced. Ifthe average grain size of the tungsten crystal grains exceeds 60 μm, thetungsten crystal grains become coarse and the processability decreases,and thus, fracture may occur in the sintered body when the sintered bodyis processed into a complex shape.

EXAMPLES

Examples 1 to 29 and Comparative Examples 1 to 13 of the presentinvention in which the flat plate-like sintered tungsten alloy isfabricated to check the effects of the aforementioned embodiment will bedescribed hereinafter.

Example 1

In this Example, the aforementioned flat plate-like sintered tungstenalloy of the present invention as an intermediate product wasfabricated. Namely, the aforementioned steps (1) to (7) were performedto fabricate the flat plate-like sintered tungsten alloy.

First, preparation was made such that a tungsten powder having anaverage grain size of 3 μm was contained at a blending ratio of 95% bymass, a nickel powder having an average grain size of 4 μm was containedat a blending ratio of 3.5% by mass, and an iron powder having anaverage grain size of 3 μm was contained at a blending ratio of 1.5% bymass ((1) materials preparing step).

Next, the tungsten powder, the nickel powder and the iron powderprepared as described above were mixed by using the Lodige mixer, toobtain a mixture ((2) mixing step).

Then, by using cold isotropic pressure press (CIP), a pressure of 196MPa was applied to the mixture obtained as described above and themixture was molded to obtain a molded body ((3) molding step). Theobtained molded body had a dimension of 176 mm×176 mm×8.2 mm.

Furthermore, the molded body obtained as described above was sintered inthe hydrogen gas atmosphere furnace at a temperature of 1460° C. for 80minutes to obtain a sintered body ((4) sintering step). The obtainedsintered body had a dimension of 150 mm×150 mm×7 mm.

FIG. 1 shows an optical microscope photograph obtained by observing across section of the flat plate-like sintered tungsten alloy obtained asdescribed above.

Thereafter, by using a forging machine, the obtained flat plate-likesintered tungsten alloy was deformed in the thickness direction at atemperature of 25° C. at a deformation rate of 30% to introduce adistortion into the sintered tungsten alloy ((5) distortion introducingstep). The flat plate-like sintered tungsten alloy having the distortionintroduced therein had a dimension of 177 mm×177 mm×5 mm.

Heat treatment was performed on the flat plate-like sintered tungstenalloy having the distortion introduced therein, in the vacuum furnace ata temperature of 1200° C. for 3 hours ((6) heat treatment step).

FIG. 2 shows an optical microscope photograph obtained by observing across section of the flat plate-like sintered tungsten alloy obtained asdescribed above. As shown in FIG. 2, it can be seen that the tungstencrystal grains have become fine grains and the average grain size isapproximately 10 μm.

Finally, a sample of the flat plate-like sintered tungsten alloysubjected to heat treatment was heated in the hydrogen gas atmospherefurnace to a temperature of 1100° C., and then, the sample was taken outfrom the furnace and rolling processing was immediately performed at arolling processing rate of approximately 10%, and the rolling processingwas repeated until the rolling processing rate in total reached 80% ((7)hot-rolling step). The flat plate-like sintered tungsten alloy afterhot-rolling processing had a dimension of 100 mm×1070 mm×1 mm.

The flat plate-like sintered tungsten alloy of the present invention asan intermediate product was thus fabricated.

Heat treatment was performed on the obtained flat plate-like sinteredtungsten alloy as an intermediate product in the vacuum furnace at atemperature of 1450° C. for 60 minutes ((8) heat treatment step).

FIG. 3 shows an optical microscope photograph obtained by observing across section of the flat plate-like sintered tungsten alloy as a finalproduct obtained as described above. As shown in FIG. 3, it can be seenthat the tungsten crystal grains have become coarse and the averagegrain size is approximately 35 μm.

Example 2

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that the heat treatmenttemperature was 1300° C. in the heat treatment step after thehot-rolling step. Similarly to Example 1, a cross section of the flatplate-like sintered tungsten alloy as a final product was observed byusing an optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 21 μm.

Example 3

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that the heat treatmenttemperature was 1550° C. in the heat treatment step after thehot-rolling step. Similarly to Example 1, a cross section of the flatplate-like sintered tungsten alloy as a final product was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 43 μm.

Example 4

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that the heat treatmenttime was 10 minutes in the heat treatment step after the hot-rollingstep. Similarly to Example 1, a cross section of the flat plate-likesintered tungsten alloy as a final product was observed by using theoptical microscope. As a result of observation, the average grain sizeof the tungsten crystal grains was approximately 25 μm.

Example 5

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that the heat treatmenttime was 3 hours in the heat treatment step after the hot-rolling step.Similarly to Example 1, a cross section of the flat plate-like sinteredtungsten alloy as a final product was observed by using the opticalmicroscope. As a result of observation, the average grain size of thetungsten crystal grains was approximately 39 μm.

Example 6

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that the heat treatmenttime was 5 hours in the heat treatment step after the hot-rolling step.Similarly to Example 1, a cross section of the flat plate-like sinteredtungsten alloy as a final product was observed by using the opticalmicroscope. As a result of observation, the average grain size of thetungsten crystal grains was approximately 57 μm.

Example 7

A flat plate-like sintered tungsten alloy having a thickness of 1.1 mmwas fabricated similarly to Example 1, except that a distortion wasintroduced into the sintered tungsten alloy by deforming the sinteredtungsten alloy in the thickness direction at a deformation rate of 20%in the distortion introducing step and the heat treatment temperaturewas 1500° C. in the heat treatment step after the hot-rolling step.Similarly to Example 1, a cross section of the flat plate-like sinteredtungsten alloy after heat treatment after the distortion introducingstep was observed by using the optical microscope. As a result ofobservation, the average grain size of the tungsten crystal grains wasapproximately 19 μm. In addition, similarly to Example 1, a crosssection of the flat plate-like sintered tungsten alloy as a finalproduct was observed by using the optical microscope. As a result ofobservation, the average grain size of the tungsten crystal grains wasapproximately 39 μm.

Example 8

A flat plate-like sintered tungsten alloy having a thickness of 0.9 mmwas fabricated similarly to Example 1, except that a distortion wasintroduced into the sintered tungsten alloy by deforming the sinteredtungsten alloy in the thickness direction at a deformation rate of 40%in the distortion introducing step and the heat treatment temperaturewas 1500° C. in the heat treatment step after the hot-rolling step.Similarly to Example 1, a cross section of the flat plate-like sinteredtungsten alloy after heat treatment after the distortion introducingstep was observed by using the optical microscope. As a result ofobservation, the average grain size of the tungsten crystal grains wasapproximately 10 μm. In addition, similarly to Example 1, a crosssection of the flat plate-like sintered tungsten alloy as a finalproduct was observed by using the optical microscope. As a result ofobservation, the average grain size of the tungsten crystal grains wasapproximately 38 μm.

Example 9

A flat plate-like sintered tungsten alloy having a thickness of 0.7 mmwas fabricated similarly to Example 1, except that a distortion wasintroduced into the sintered tungsten alloy by deforming the sinteredtungsten alloy in the thickness direction at a deformation rate of 50%in the distortion introducing step and the heat treatment temperaturewas 1500° C. in the heat treatment step after the hot-rolling step.Similarly to Example 1, a cross section of the flat plate-like sinteredtungsten alloy after heat treatment after the distortion introducingstep was observed by using the optical microscope. As a result ofobservation, the average grain size of the tungsten crystal grains wasapproximately 5 μm. In addition, similarly to Example 1, a cross sectionof the flat plate-like sintered tungsten alloy as a final product wasobserved by using the optical microscope. As a result of observation,the average grain size of the tungsten crystal grains was approximately39 μm.

Example 10

A flat plate-like sintered tungsten alloy having a thickness of 2.0 mmwas fabricated similarly to Example 1, except that rolling processingwas performed until the rolling processing rate in total reached 60% inthe hot-rolling step. Similarly to Example 1, a cross section of theflat plate-like sintered tungsten alloy as a final product was observedby using the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 39 μm.

Example 11

A flat plate-like sintered tungsten alloy having a thickness of 1.5 mmwas fabricated similarly to Example 1, except that rolling processingwas performed until the rolling processing rate in total reached 70% inthe hot-rolling step. Similarly to Example 1, a cross section of theflat plate-like sintered tungsten alloy as a final product was observedby using the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 36 μm.

Example 12

A flat plate-like sintered tungsten alloy having a thickness of 0.7 mmwas fabricated similarly to Example 1, except that rolling processingwas performed until the rolling processing rate in total reached 85% inthe hot-rolling step. Similarly to Example 1, a cross section of theflat plate-like sintered tungsten alloy as a final product was observedby using the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 37 μm.

Example 13

A flat plate-like sintered tungsten alloy having a thickness of 0.5 mmwas fabricated similarly to Example 1, except that rolling processingwas performed until the rolling processing rate in total reached 90% inthe hot-rolling step. Similarly to Example 1, a cross section of theflat plate-like sintered tungsten alloy as a final product was observedby using the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 37 μm.

Example 14

A flat plate-like sintered tungsten alloy having a thickness of 0.5 mmwas fabricated similarly to Example 1, except that the thickness of asintered body obtained in the sintering step was 14 mm and rollingprocessing was performed until the rolling processing rate in totalreached 95% in the hot-rolling step. Similarly to Example 1, a crosssection of the flat plate-like sintered tungsten alloy as a finalproduct was observed by using the optical microscope. As a result ofobservation, the average grain size of the tungsten crystal grains wasapproximately 39 μm.

Example 15

A flat plate-like sintered tungsten alloy having a thickness of 0.5 mmwas fabricated similarly to Example 1, except that the thickness of asintered body obtained in the sintering step was 20 mm, a distortion wasintroduced into the sintered tungsten alloy by deforming the sinteredtungsten alloy in the thickness direction at a deformation rate of 50%in the distortion introducing step, rolling processing was performeduntil the rolling processing rate in total reached 95% in thehot-rolling step, and the heat treatment temperature was 1300° C. in theheat treatment step after the hot-rolling step. Similarly to Example 1,a cross section of the flat plate-like sintered tungsten alloy afterheat treatment after the distortion introducing step was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 5 μm. Inaddition, similarly to Example 1, a cross section of the flat plate-likesintered tungsten alloy as a final product was observed by using theoptical microscope. As a result of observation, the average grain sizeof the tungsten crystal grains was approximately 23 μm.

Example 16

A flat plate-like sintered tungsten alloy having a thickness of 0.5 mmwas fabricated similarly to Example 1, except that the thickness of asintered body obtained in the sintering step was 20 mm, a distortion wasintroduced into the sintered tungsten alloy by deforming the sinteredtungsten alloy in the thickness direction at a deformation rate of 50%in the distortion introducing step, rolling processing was performeduntil the rolling processing rate in total reached 95% in thehot-rolling step, and the heat treatment temperature was 1450° C. in theheat treatment step after the hot-rolling step. Similarly to Example 1,a cross section of the flat plate-like sintered tungsten alloy afterheat treatment after the distortion introducing step was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 5 μm. Inaddition, similarly to Example 1, a cross section of the flat plate-likesintered tungsten alloy as a final product was observed by using theoptical microscope. As a result of observation, the average grain sizeof the tungsten crystal grains was approximately 37 μm.

Example 17

A flat plate-like sintered tungsten alloy having a thickness of 0.5 mmwas fabricated similarly to Example 1, except that the thickness of asintered body obtained in the sintering step was 20 mm, a distortion wasintroduced into the sintered tungsten alloy by deforming the sinteredtungsten alloy in the thickness direction at a deformation rate of 50%in the distortion introducing step, rolling processing was performeduntil the rolling processing rate in total reached 95% in thehot-rolling step, and the heat treatment temperature was 1550° C. in theheat treatment step after the hot-rolling step. Similarly to Example 1,a cross section of the flat plate-like sintered tungsten alloy afterheat treatment after the distortion introducing step was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 5 μm. Inaddition, similarly to Example 1, a cross section of the flat plate-likesintered tungsten alloy as a final product was observed by using theoptical microscope. As a result of observation, the average grain sizeof the tungsten crystal grains was approximately 47 μm.

Example 18

A flat plate-like sintered tungsten alloy having a thickness of 2.2 mmwas fabricated similarly to Example 1, except that a tungsten powderhaving an average grain size of 1 μm was used and a copper powder wasused instead of the iron powder in the materials preparing step, adistortion was introduced into the sintered tungsten alloy by deformingthe sintered tungsten alloy in the thickness direction at a deformationrate of 20% in the distortion introducing step, rolling processing wasperformed until the rolling processing rate in total reached 60% in thehot-rolling step, and the heat treatment temperature was 1300° C. in theheat treatment step after the hot-rolling step. Similarly to Example 1,a cross section of the flat plate-like sintered tungsten alloy afterheat treatment after the distortion introducing step was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 19 μm. Inaddition, similarly to Example 1, a cross section of the flat plate-likesintered tungsten alloy as a final product was observed by using theoptical microscope. As a result of observation, the average grain sizeof the tungsten crystal grains was approximately 21 μm.

Example 19

A flat plate-like sintered tungsten alloy having a thickness of 1.7 mmwas fabricated similarly to Example 1, except that a tungsten powderhaving an average grain size of 5 μm was used and a copper powder wasused instead of the iron powder in the materials preparing step, adistortion was introduced into the sintered tungsten alloy by deformingthe sintered tungsten alloy in the thickness direction at a deformationrate of 20% in the distortion introducing step, rolling processing wasperformed until the rolling processing rate in total reached 70% in thehot-rolling step, and the heat treatment temperature was 1300° C. in theheat treatment step after the hot-rolling step. Similarly to Example 1,a cross section of the flat plate-like sintered tungsten alloy afterheat treatment after the distortion introducing step was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 19 μm. Inaddition, similarly to Example 1, a cross section of the flat plate-likesintered tungsten alloy as a final product was observed by using theoptical microscope. As a result of observation, the average grain sizeof the tungsten crystal grains was approximately 23 μm.

Example 20

A flat plate-like sintered tungsten alloy having a thickness of 0.7 mmwas fabricated similarly to Example 1, except that a tungsten powderhaving an average grain size of 10 μm was used and a copper powder wasused instead of the iron powder in the materials preparing step, adistortion was introduced into the sintered tungsten alloy by deformingthe sintered tungsten alloy in the thickness direction at a deformationrate of 20% in the distortion introducing step, rolling processing wasperformed until the rolling processing rate in total reached 90% in thehot-rolling step, and the heat treatment temperature was 1300° C. in theheat treatment step after the hot-rolling step. Similarly to Example 1,a cross section of the flat plate-like sintered tungsten alloy afterheat treatment after the distortion introducing step was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 19 μm. Inaddition, similarly to Example 1, a cross section of the flat plate-likesintered tungsten alloy as a final product was observed by using theoptical microscope. As a result of observation, the average grain sizeof the tungsten crystal grains was approximately 22 μm.

Example 21

A flat plate-like sintered tungsten alloy having a thickness of 0.5 mmwas fabricated similarly to Example 1, except that preparation was madein the materials preparing step to contain a tungsten powder at ablending ratio of 85% by mass, a nickel powder at a blending ratio of10.5% by mass, and an iron powder at a blending ratio of 4.5% by mass,rolling processing was performed until the rolling processing rate intotal reached 90% in the hot-rolling step, and the heat treatmenttemperature was 1300° C. and the heat treatment time was 3 hours in theheat treatment step after the hot-rolling step. Similarly to Example 1,a cross section of the flat plate-like sintered tungsten alloy afterheat treatment after the distortion introducing step was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 9 μm. Inaddition, similarly to Example 1, a cross section of the flat plate-likesintered tungsten alloy as a final product was observed by using theoptical microscope. As a result of observation, the average grain sizeof the tungsten crystal grains was approximately 24 μm.

Example 22

A flat plate-like sintered tungsten alloy having a thickness of 0.5 mmwas fabricated similarly to Example 1, except that preparation was madein the materials preparing step to contain a tungsten powder at ablending ratio of 90% by mass, a nickel powder at a blending ratio of 7%by mass, and an iron powder at a blending ratio of 3% by mass, rollingprocessing was performed until the rolling processing rate in totalreached 90% in the hot-rolling step, and the heat treatment temperaturewas 1300° C. and the heat treatment time was 3 hours in the heattreatment step after the hot-rolling step. Similarly to Example 1, across section of the flat plate-like sintered tungsten alloy after heattreatment after the distortion introducing step was observed by usingthe optical microscope. As a result of observation, the average grainsize of the tungsten crystal grains was approximately 10 μm. Inaddition, similarly to Example 1, a cross section of the flat plate-likesintered tungsten alloy as a final product was observed by using theoptical microscope. As a result of observation, the average grain sizeof the tungsten crystal grains was approximately 24 μm.

Example 23

A flat plate-like sintered tungsten alloy having a thickness of 0.5 mmwas fabricated similarly to Example 1, except that preparation was madein the materials preparing step to contain a tungsten powder at ablending ratio of 98% by mass, a nickel powder at a blending ratio of1.4% by mass, and an iron powder at a blending ratio of 0.6% by mass,rolling processing was performed until the rolling processing rate intotal reached 90% in the hot-rolling step, and the heat treatmenttemperature was 1300° C. and the heat treatment time was 3 hours in theheat treatment step after the hot-rolling step. Similarly to Example 1,a cross section of the flat plate-like sintered tungsten alloy afterheat treatment after the distortion introducing step was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 10 μm. Inaddition, similarly to Example 1, a cross section of the flat plate-likesintered tungsten alloy as a final product was observed by using theoptical microscope. As a result of observation, the average grain sizeof the tungsten crystal grains was approximately 26 μm.

Example 24

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that a nickel powderhaving an average grain size of 1 μm was used and a cobalt powder wasused instead of the iron powder in the materials preparing step, and theheat treatment temperature was 1300° C. and the heat treatment time was5 hours in the heat treatment step after the hot-rolling step. Similarlyto Example 1, a cross section of the flat plate-like sintered tungstenalloy after heat treatment after the distortion introducing step wasobserved by using the optical microscope. As a result of observation,the average grain size of the tungsten crystal grains was approximately11 μm. In addition, similarly to Example 1, a cross section of the flatplate-like sintered tungsten alloy as a final product was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 28 μm.

Example 25

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that a nickel powderhaving an average grain size of 5 μm was used and a cobalt powder wasused instead of the iron powder in the materials preparing step, and theheat treatment temperature was 1300° C. and the heat treatment time was5 hours in the heat treatment step after the hot-rolling step. Similarlyto Example 1, a cross section of the flat plate-like sintered tungstenalloy after heat treatment after the distortion introducing step wasobserved by using the optical microscope. As a result of observation,the average grain size of the tungsten crystal grains was approximately10 μm. In addition, similarly to Example 1, a cross section of the flatplate-like sintered tungsten alloy as a final product was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 30 μm.

Example 26

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that a nickel powderhaving an average grain size of 10 μm was used and a cobalt powder wasused instead of the iron powder in the materials preparing step, and theheat treatment temperature was 1300° C. and the heat treatment time was5 hours in the heat treatment step after the hot-rolling step. Similarlyto Example 1, a cross section of the flat plate-like sintered tungstenalloy after heat treatment after the distortion introducing step wasobserved by using the optical microscope. As a result of observation,the average grain size of the tungsten crystal grains was approximately10 μm. In addition, similarly to Example 1, a cross section of the flatplate-like sintered tungsten alloy as a final product was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 27 μm.

Example 27

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that an iron powder havingan average grain size of 1 μm was used in the materials preparing step,a distortion was introduced into the sintered tungsten alloy bydeforming the sintered tungsten alloy in the thickness direction at adeformation rate of 50% in the distortion introducing step, and rollingprocessing was performed until the rolling processing rate in totalreached 60% in the hot-rolling step. Similarly to Example 1, a crosssection of the flat plate-like sintered tungsten alloy after heattreatment after the distortion introducing step was observed by usingthe optical microscope. As a result of observation, the average grainsize of the tungsten crystal grains was approximately 5 μm. In addition,similarly to Example 1, a cross section of the flat plate-like sinteredtungsten alloy as a final product was observed by using the opticalmicroscope. As a result of observation, the average grain size of thetungsten crystal grains was approximately 32 μm.

Example 28

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that an iron powder havingan average grain size of 5 μm was used in the materials preparing step,a distortion was introduced into the sintered tungsten alloy bydeforming the sintered tungsten alloy in the thickness direction at adeformation rate of 50% in the distortion introducing step, and rollingprocessing was performed until the rolling processing rate in totalreached 60% in the hot-rolling step. Similarly to Example 1, a crosssection of the flat plate-like sintered tungsten alloy after heattreatment after the distortion introducing step was observed by usingthe optical microscope. As a result of observation, the average grainsize of the tungsten crystal grains was approximately 5 μm. In addition,similarly to Example 1, a cross section of the flat plate-like sinteredtungsten alloy as a final product was observed by using the opticalmicroscope. As a result of observation, the average grain size of thetungsten crystal grains was approximately 35 μm.

Example 29

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that an iron powder havingan average grain size of 10 μm was used in the materials preparing step,a distortion was introduced into the sintered tungsten alloy bydeforming the sintered tungsten alloy in the thickness direction at adeformation rate of 50% in the distortion introducing step, and rollingprocessing was performed until the rolling processing rate in totalreached 60% in the hot-rolling step. Similarly to Example 1, a crosssection of the flat plate-like sintered tungsten alloy after heattreatment after the distortion introducing step was observed by usingthe optical microscope. As a result of observation, the average grainsize of the tungsten crystal grains was approximately 5 μm. In addition,similarly to Example 1, a cross section of the flat plate-like sinteredtungsten alloy as a final product was observed by using the opticalmicroscope. As a result of observation, the average grain size of thetungsten crystal grains was approximately 33 μm.

Comparative Example 1

A conventional sintered tungsten alloy fabricated by performing theaforementioned steps (1) to (4) was processed to have a thickness of 1mm by grinding and polishing.

Comparative Example 2

On a conventional sintered tungsten alloy fabricated by performing theaforementioned steps (1) to (4), the steps (5) and (6) were notperformed and (7) hot-rolling step was performed until the rollingprocessing rate reached 60% (thickness became 2 mm) immediately aftersintering. Thereafter, the sintered tungsten alloy subjected to rollingprocessing was processed to have a thickness of 1 mm by grinding andpolishing.

FIG. 4 shows an optical microscope photograph obtained by observing across section of the flat plate-like sintered tungsten alloy obtained asdescribed above.

Comparative Example 3

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that the heat treatmenttemperature was 1200° C. in the heat treatment step after thehot-rolling step. Similarly to Example 1, a cross section of the flatplate-like sintered tungsten alloy as a final product was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 18 μm.

Comparative Example 4

An attempt was made to fabricate a flat plate-like sintered tungstenalloy having a thickness of 1.0 mm similarly to Example 1, except thatthe heat treatment temperature was 1600° C. in the heat treatment stepafter the hot-rolling step. However, the flat plate-like sinteredtungsten alloy melted.

Comparative Example 5

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that the heat treatmenttime was 6 minutes in the heat treatment step after the hot-rollingstep. Similarly to Example 1, a cross section of the flat plate-likesintered tungsten alloy as a final product was observed by using theoptical microscope. As a result of observation, the average grain sizeof the tungsten crystal grains was approximately 17 μm.

Comparative Example 6

A flat plate-like sintered tungsten alloy having a thickness of 1.0 mmwas fabricated similarly to Example 1, except that the heat treatmenttime was 6 hours in the heat treatment step after the hot-rolling step.Similarly to Example 1, a cross section of the flat plate-like sinteredtungsten alloy as a final product was observed by using the opticalmicroscope. As a result of observation, the average grain size of thetungsten crystal grains was approximately 64 μm.

Comparative Example 7

An attempt was made to fabricate a flat plate-like sintered tungstenalloy similarly to Example 1, except that a distortion was introducedinto the sintered tungsten alloy by deforming the sintered tungstenalloy in the thickness direction at a deformation rate of 17% in thedistortion introducing step and the heat treatment temperature was 1500°C. in the heat treatment step after the hot-rolling step. However,fracture occurred after the hot-rolling step. Similarly to Example 1, across section of the flat plate-like sintered tungsten alloy after heattreatment after the distortion introducing step was observed by usingthe optical microscope. As a result of observation, the average grainsize of the tungsten crystal grains was approximately 24.3 μm.

Comparative Example 8

An attempt was made to fabricate a flat plate-like sintered tungstenalloy similarly to Example 1, except that a distortion was introducedinto the sintered tungsten alloy by deforming the sintered tungstenalloy in the thickness direction at a deformation rate of 60% in thedistortion introducing step and the heat treatment temperature was 1500°C. in the heat treatment step after the hot-rolling step. However,fracture occurred in the distortion introducing step.

Comparative Example 9

A flat plate-like sintered tungsten alloy having a thickness of 2.5 mmwas fabricated similarly to Example 1, except that rolling processingwas performed until the rolling processing rate in total reached 50% inthe hot-rolling step. Similarly to Example 1, a cross section of theflat plate-like sintered tungsten alloy as a final product was observedby using the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 37 μm.

Comparative Example 10

An attempt was made to fabricate a flat plate-like sintered tungstenalloy similarly to Example 1, except that rolling processing wasperformed until the rolling processing rate in total reached 97% in thehot-rolling step. However, fracture occurred after the hot-rolling step.

Comparative Example 11

A flat plate-like sintered tungsten alloy having a thickness of 0.5 mmwas fabricated similarly to Example 1, except that the thickness of asintered body obtained in the sintering step was 20 mm, a distortion wasintroduced into the sintered tungsten alloy by deforming the sinteredtungsten alloy in the thickness direction at a deformation rate of 50%in the distortion introducing step, rolling processing was performeduntil the rolling processing rate in total reached 95% in thehot-rolling step, and the heat treatment temperature was 1200° C. in theheat treatment step after the hot-rolling step. Similarly to Example 1,a cross section of the flat plate-like sintered tungsten alloy afterheat treatment after the distortion introducing step was observed byusing the optical microscope. As a result of observation, the averagegrain size of the tungsten crystal grains was approximately 5 μm. Inaddition, similarly to Example 1, a cross section of the flat plate-likesintered tungsten alloy as a final product was observed by using theoptical microscope. As a result of observation, the average grain sizeof the tungsten crystal grains was approximately 17 μm.

Comparative Example 12

An attempt was made to fabricate a flat plate-like sintered tungstenalloy having a thickness of 0.5 mm similarly to Example 1, except thatthe thickness of a sintered body obtained in the sintering step was 20mm, a distortion was introduced into the sintered tungsten alloy bydeforming the sintered tungsten alloy in the thickness direction at adeformation rate of 50% in the distortion introducing step, rollingprocessing was performed until the rolling processing rate in totalreached 95% in the hot-rolling step, and the heat treatment temperaturewas 1600° C. in the heat treatment step after the hot-rolling step.However, the flat plate-like sintered tungsten alloy melted. Similarlyto Example 1, a cross section of the flat plate-like sintered tungstenalloy after heat treatment after the distortion introducing step wasobserved by using the optical microscope. As a result of observation,the average grain size of the tungsten crystal grains was approximately5 μm.

Comparative Example 13

When (7) hot-rolling step was performed on a conventional sinteredtungsten alloy fabricated by performing the aforementioned steps (1) to(4) in Example 1, until the rolling processing rate reached 65%immediately after sintering, fracture occurred in the sintered tungstenalloy.

(Measurement of Average Thickness and Average Length of Tungsten CrystalGrains)

As shown in FIG. 5, by using a scanning electron microscope (SEM),observation was conducted of a first cross section 101 along thedirection of a thickness T₀ (1 mm) orthogonal to the extending directionof a plane 100 of a flat plate-like sintered tungsten alloy 1 obtainedas an intermediate product in each of Examples 1 to 29 and ComparativeExamples 1 to 6, 9, 11 and 12, and a second cross section 102 which is across section along the direction of thickness T₀ orthogonal to theextending direction of plane 100 of flat plate-like sintered tungstenalloy 1 and which is orthogonal to the first cross section.

Specifically, in each of first cross section 101 and second crosssection 102, a photograph of an arbitrary site (field of view) was takenat a magnification of 1000×, and a photograph of a site (field of view)displaced in this cross section from the arbitrary site along theextending direction of plane 100 was taken. 7 photographs of fields ofview obtained by sequential displacement were connected in the extendingdirection of plane 100 to obtain a photograph of a cross section portionhaving a thickness T of 70 μm and a width W of 500 μm. There were thusobtained photographs of a first cross section portion of certain width W(500 μm) and certain thickness T (70 μm) selected from first crosssection 101 and a second cross section portion of certain width W (500μm) and certain thickness T (70 μm) selected from second cross section102. FIG. 6 shows one example (Example 1) of the scanning electronmicroscope (SEM) photograph of the cross section portion.

FIG. 7 schematically shows the aforementioned cross section portion. Asshown in FIG. 7, in the photograph of each of first cross sectionportion 101 a and second cross section portion 102 a obtained asdescribed above, measurement was conducted of thicknesses t and lengthss of a plurality of tungsten crystal grains G1 to G4 which intersectwith a center line 200 passing through a center of certain width W (500μm) and extending in the direction of certain thickness T (70 μm).Average values of these measurement values were obtained and defined asthe average thickness and the average length of the tungsten crystalgrains. A ratio of the average length to the average thickness of thetungsten crystal grains was also calculated.

Table 1 shows the average thickness and the average length as well asthe ratio of the average length to the average thickness of the tungstencrystal grains calculated as described above.

(Measurement of Elongation Percentage of Sintered Tungsten Alloy)

As shown in FIG. 8, a tensile test piece 10 having thickness T wasfabricated from the flat plate-like sintered tungsten alloy obtained asa final product in each of Examples 1 to 29 and Comparative Examples 1to 3, 5, 6, 9, 11 and 12. The gauge length was set at 8 mm with centerline 20 centered.

Tensile test piece 10 thus fabricated was placed in a tensile testmachine of model No. 5867 manufactured by Instron Co., Ltd. and atensile test was conducted in the ambient atmosphere at a testtemperature of 20° C. at a tensile speed of 300 mm/min. until ruptureoccurred. A rate of increase in gauge length of the test piece untilrupture occurred was defined as the elongation percentage. As to theflat plate-like sintered tungsten alloy obtained as an intermediateproduct in Example 1 as well, the elongation percentage was measuredsimilarly to the above.

Table 1 shows the measurement result of the elongation percentageobtained as described above. A parenthesized numerical value in “Example1” in Table 1 represents the elongation percentage of the flatplate-like sintered tungsten alloy obtained as an intermediate productin Example 1.

(Measurement of X-Ray Diffraction Intensity Ratio of (111) Plane ofNi—(Fe, Cu, Co) Phase) A test piece having a thickness of 0.5 mm andhaving a plane of 8 mm×8 mm was fabricated from the flat plate-likesintered tungsten alloy obtained as a final product in each of Examples1 to 29 and Comparative Examples 1 to 3, 5, 6, 9, 11 and 12.

The plane of 8 mm×8 mm of the fabricated test piece was irradiated withX rays by using an X-ray diffraction device of model No.SmartLab-2D-PILATUS manufactured by Rigaku Corporation, to measure X-raydiffraction intensities of a (111) plane, a (100) plane, a (110) plane,and a (311) plane of a Ni—(Fe, Cu, Co) phase in the flat plate surface.The X-ray diffraction conditions were such that the used X rays wereCu-Kα, excitation conditions were 45 kV and 200 mA, a collimator of 40.8mm was used, and a measurement method was a θ-2θ method. The plane of 8mm×8 mm serving as the measurement plane was subjected to mechanicalpolishing and then alkaline electrolytic polishing.

Based on the obtained values of the X-ray diffraction intensities of therespective planes, an X-ray diffraction intensity ratio of the (111)plane (assuming that the X-ray diffraction intensities of the (111)plane, the (100) plane, the (110) plane, and the (311) plane are I(111),I(100), I(110), and I(311), respectively, the X-ray diffractionintensity ratio of the (111) plane is a value of[I(111)/I(111)+I(100)+I(110)+I(311))]) was calculated.

Table 1 shows the measurement result of the X-ray diffraction intensityratio of the (111) plane of the Ni—(Fe, Cu, Co) phase obtained asdescribed above.

TABLE 1 Tungsten crystal grains (intermediate product) Ratio X-ray dif-(average fraction Average Average length/ Elongation intensity thicknesslength average percentage ratio of Sample No. [μm] [μm] thickness) [%](111) plane Example 1 5.38 97.42 18.11 27.7(1.0) 0.75 Example 2 5.3897.42 18.11 21.1 0.73 Example 3 5.38 97.42 18.11 29.7 0.69 Example 45.38 97.42 18.11 22.9 0.77 Example 5 5.38 97.42 18.11 27.5 0.74 Example6 5.38 97.42 18.11 23.6 0.71 Example 7 5.81 108.97 18.76 28.6 0.76Example 8 4.23 94.63 22.37 29.2 0.79 Example 9 3.77 85.71 22.73 29.10.79 Example 10 5.88 54.78 9.32 22.3 0.70 Example 11 5.49 68.23 12.4326.4 0.72 Example 12 4.02 97.46 24.24 32.7 0.75 Example 13 2.85 183.7864.48 38.7 0.78 Example 14 2.33 227.85 97.79 41.5 0.84 Example 15 2.02249.24 123.39 22.7 0.89 Example 16 2.02 249.24 123.39 44.3 0.88 Example17 2.02 249.24 123.39 32.1 0.76 Example 18 9.69 90.50 9.34 20.5 0.68Example 19 6.34 98.45 15.53 26.6 0.71 Example 20 3.01 185.47 61.62 32.40.76 Example 21 4.72 99.72 21.13 23.9 0.78 Example 22 4.98 102.89 20.6624.5 0.78 Example 23 5.37 93.58 17.43 23.7 0.73 Example 24 5.42 98.2518.13 24.5 0.76 Example 25 5.32 99.62 18.73 25.1 0.78 Example 26 5.4796.31 17.61 24.1 0.74 Example 27 2.89 30.87 10.68 23.9 0.68 Example 282.94 31.78 10.81 22.7 0.71 Comparative 2.93 30.94 10.56 23.7 0.70Example 29 Comparative — — — 18.4 0.57 Example 1 Comparative 14.00 62.004.43 0.9 0.60 Example 2 Comparative 5.38 97.42 18.11 14.5 0.71 Example 3Comparative 5.38 97.42 18.11 — — Example 4 Comparative 5.38 97.42 18.117.7 0.81 Example 5 Comparative 5.38 97.42 18.11 17.2 0.63 Example 6Comparative 6.22 25.71 4.13 18.7 0.63 Example 9 Comparative 2.02 249.24123.39 13.7 0.91 Example 11 Comparative 2.02 249.24 123.39 — — Example12

From Table 1, it can be seen that the sample in each Example of thepresent invention exhibits a high elongation percentage.

It can also be seen that a value of the X-ray diffraction intensityratio of the (111) plane of the Ni—(Fe, Cu, Co) phase is 0.68 or moreand 0.9 or less in the sample in each Example of the present invention.

It should be understood that the embodiments and the examples disclosedherein are illustrative and not limitative in any respect. The scope ofthe present invention is defined by the terms of the claims, rather thanthe embodiments and the examples above, and is intended to include anychanges and modifications within the scope and meaning equivalent to theterms of the claims.

INDUSTRIAL APPLICABILITY

The sintered tungsten alloy of the present invention is used as aradiation shield material in radiation medical devices, nuclearreactor-related devices and the like.

REFERENCE SIGNS LIST

1 sintered tungsten alloy; 100 plane; 101 first cross section; 101 afirst cross section portion; 102 second cross section; 102 a secondcross section portion; 200 center line; G1 to G4 tungsten crystal grain;T, T₀ thickness; W width; t thickness; s length.

1-3. (canceled)
 4. A flat plate-like sintered tungsten alloy, containing85% by mass or more and 98% by mass or less of tungsten, 1.4% by mass ormore and 11% by mass or less of nickel, and 0.6% by mass or more and 6%by mass or less of at least one substance selected from the groupconsisting of iron, copper and cobalt, wherein an elongation percentageof the flat plate-like sintered tungsten alloy in a planar direction is20% or more; and an X-ray diffraction intensity ratio of a (111) planeof a Ni—(Fe, Cu, Co) phase in a flat plate surface of the flatplate-like sintered tungsten alloy (assuming that X-ray diffractionintensities of the (111) plane, a (100) plane, a (110) plane, and a(311) plane are I(111), I(100), I(110), and I(311), respectively, theX-ray diffraction intensity ratio of the (111) plane is a value of[I(111)/{I(111)+I(100)+I(110)+I(311)}]) is 0.68 or more and 0.9 or less.5. The sintered tungsten alloy according to claim 4, wherein a thicknessof the flat plate-like sintered tungsten alloy is 1.5 mm or smaller.